Compact structure of gas sensor and production method thereof

ABSTRACT

An improved structure of a gas sensor is provided. The gas sensor includes a hollow housing in which a gas sensing element is disposed, a protective cover installed on an end of the housing, a metallic cover installed on the other end of the housing, and an insulator in which end portions of leads connecting with the gas sensing element are disposed. The insulator is retained at a flange thereof in the housing elastically using a washer. The protective cover is welded at an end thereof to the whole circumference of the end of the housing.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to a gas sensor which may beemployed in an air-fuel ratio control system for automotive vehicles formeasuring the concentration of gas such as O₂, NOx, or CO, and moreparticularly to an improved compact structure of such a gas sensor and aproduction method thereof.

2. Background Art

It is known that control of burning of an internal combustion engines asa function of the concentration of oxygen contained in exhaust gassesand the air-fuel ratio of a mixture is effective in the energy savingand the emission control. As one of gas sensors suitable for measuringthe concentration of oxygen in exhaust gasses, a gas sensor including asolid electrolyte body made of zirconia is known. This type of gassensor is, however, required to be reduced in size and production costsand improved in durability and reliability. The approach to satisfactionof these requirements has still left room for improvement.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to provide agas sensor designed to meet the above discussed requirements in aconventional structure of a gas sensor.

According to one aspect of the invention, there is provided a gas sensorwhich comprises: (a) a gas sensing element having an gas-exposedportion; (b) a hollow housing having a first and a second end, thehousing holding the gas sensing element therein so as to arrange thegas-exposed portion outside the housing for exposure to a gas to bemeasured; (c) a protective cover installed on the first end of thehousing to cover the gas-exposed portion of the gas sensing element; (d)leads connecting with the gas sensing element, extending from the secondend of the housing for electrical communication with an external device;(e) a metallic cover installed on the second end of the housing, themetallic cover including a small-diameter portion, a large-diameterportion, and a shoulder portion connecting the smaller-diameter portionand the large-diameter portion; and (f) an insulator including a bodyand a flange projecting from the body, having disposed therein the leadsconnecting with the gas sensing element, the body having an outerdiameter smaller than an inner diameter of the small-diameter portion ofthe metallic cover and being disposed within the small-diameter portion,the flange having a first and a second end surface opposed to each otherand an outer diameter which is smaller than an inner diameter of thelarge-diameter portion of the metallic cover and which is greater thanthe inner diameter of the small-diameter portion, the insulator beingdisposed in the metallic cover with the first end surface of the flangeurged by an elastic member to bring the second end surface into constantengagement with an inner wall of the shoulder portion of the metalliccover.

In the preferred mode of the invention, the metallic cover has a givenlength. The elastic member is so designed as to produce a first pressureacting on an inner wall of the large-diameter portion of the metalliccover in a radial direction of the large-diameter portion and a secondpressure acting on the second end surface of the flange of the insulatorin a lengthwise direction of the metallic cover perpendicular to theradial direction of the metallic cover.

The elastic member includes an annular plate and tabs. The annular platehas a diameter smaller than the inner diameter of the large-diameterportion of the metallic cover. The tabs projects from the annular plateso as to establish elastic engagement with the inner wall of thelarge-diameter portion of the metallic cover.

The elastic member also include guide protrusions each of which isdisposed between adjacent two of the tabs and which projects from theannular plate to a circular line smaller than the inner diameter of thelarge-diameter portion of the metallic cover.

The elastic member may be made of a plate member having opposed surfaceswhich are symmetrical with each other.

The tabs of the elastic member may be so designed that when theinsulator is inserted into the metallic cover, some of the tabs are bentelastically in a first direction away from one of the opposed surfacesof the annular plate by elastic pressure produced by the insertion ofthe insulator, while the other tabs are bent elastically in a seconddirection opposite the first direction.

The tabs may extend from the annular plate at an angle of approximately45° or more to one of the opposed surfaces of the annular plate.

The elastic member may also include a protrusion formed on one of theopposed surfaces thereof.

At least one of the opposed surfaces of the elastic member is painted sothat the opposed surfaces have different colors.

The tabs of the elastic member may alternatively be so designed thatwhen the insulator is inserted into the metallic cover, the tabs arebent elastically in the same direction away from one of the opposedsurfaces of the annular plate by the elastic pressure produced by theinsertion of the insulator.

An elastic insulating member is disposed on an end of the metallic coverremote from the housing to retain the leads therein. If an outerdiameter of the elastic insulating member is defined as E, and an outerdiameter of the insulator is defined as F, then E≧F.

According to the second aspect of the invention, there is provided a gassensor which comprises: (a) a gas sensing element having an gas-exposedportion; (b) a hollow housing having a first and a second end, thehousing holding the gas sensing element therein so as to arrange thegas-exposed portion outside the housing for exposure to a gas to bemeasured; (c) a protective cover installed on the first end of thehousing to cover the gas-exposed portion of the gas sensing element; (d)leads connecting with the gas sensing element, extending from the secondend of the housing for electrical communication with an external device;(e) an insulator retaining therein the leads connecting with the gassensing element; and (f) a metallic cover joined directly to the housingto hold the insulator therein.

In the preferred mode of the invention, the housing has an outer wallextending between the first and second ends. The metallic cover iswelded to the whole of a circumference of the outer wall of the housing.

A welded portion is formed with welding of the metallic cover and thehousing which includes a wider portion formed in the metallic cover anda narrower portion formed in the outer wall of the housing. If maximumwidths of the wider and narrower portions are defined as A and B,respectively, the depth of the narrower portion is defined as D, and thethickness of the metallic cover 3 defined as T, conditions of B≧0.6 Aand D≧T are satisfied.

An elastic insulating member is disposed on an end of the metallic coverremote from the housing to retain the leads therein. If an outerdiameter of the elastic insulating member is defined as E, and an outerdiameter of the insulator is defined as F, then E≧F.

According to the third aspect of the invention, there is provided a gassensor which comprises: (a) a gas sensing element having an gas-exposedportion; (b) a hollow housing having a first and a second end, thehousing holding the gas sensing element therein so as to arrange thegas-exposed portion outside the housing for exposure to a gas to bemeasured; (c) a protective cover installed on the first end of thehousing to cover the gas-exposed portion of the gas sensing element; (d)leads connecting with the gas sensing element, extending from the secondend of the housing for electrical communication with an external device;(e) an insulator retaining therein the leads connecting with the gassensing element; and (f) a metallic cover having a given length, themetallic cover being joined to the housing to hold the insulator thereinin engagement of an end of the metallic cover to the housing.

An elastic insulating member is disposed on an end of the metallic coverremote from the housing to retain the leads therein. If an outerdiameter of the elastic insulating member is defined as E, and an outerdiameter of the insulator is defined as F, then E≧F.

According to the fourth aspect of the invention, there is provided a gassensor which comprises: (a) a gas sensing element having an gas-exposedportion; (b) a hollow housing having a first and a second end, thehousing holding the gas sensing element therein so as to arrange thegas-exposed portion outside the housing for exposure to a gas to bemeasured; (c) a protective cover installed on the first end of thehousing to cover the gas-exposed portion of the gas sensing element; (d)leads connecting with the gas sensing element, extending from the secondend of the housing for electrical communication with an external device;(e) an insulator retaining therein the leads connecting with the gassensing element; and (f) a metallic cover joined to the housing to holdthe insulator therein. The hollow housing has an inner chamber and anopen end. The open end is crimped to elastically press a metal ring, asealing member, and an insulating member disposed within the innerchamber to hold the gas sensing element in the inner chamber.

The housing has formed on the inner chamber a step. The gas sensingelement has a protrusion which is urged elastically through the metalring, the sealing member, and the insulating member by crimping the openend of the housing into constant engagement with the step of thehousing.

The metal ring is made of a given length of a round bar which is looped.

The gas sensing element is made of a cup-shaped member having formedtherein a chamber and has a platinum-made outer electrode formed on thegas-exposed portion and a platinum-made inner electrode formed on aninner wall thereof. A heater is disposed in the chamber of the gassensing element.

A spring steel-made outer terminal is electrically connected to theouter electrode of the gas sensing element. A spring steel-made innerterminal is electrically connected to the inner electrode. The outerterminal has a conductive extension which is connected to the endportion of one of the leads within the insulator. The inner terminal hasa conductive extension which is connected to the end portion of theother of the leads within the insulator and a heater-holding portionwhich holds the heater in the chamber of the gas sensing element.

The heater is formed with a plate heater which has a rectangular crosssection and which is made of a lamination of a substrate formed with aceramic sheet and a heat generating member.

The housing has an annular groove formed in the first end and an annularskirt extending from the first end around an outer circumference of theannular groove. The protective cover is made of an assembly of an outercylindrical member and an inner cylindrical member. The outer and innercylindrical members has flanges which are fitted in the annular grooveof the housing. The annular shirt of the housing is crimped inward toelastically press the flanges of the outer and inner cylindrical memberstogether within the annular groove to join the metallic cover to thehousing.

An elastic insulating member is disposed on an end of the metallic coverremote from the housing to retain the leads therein. If an outerdiameter of the elastic insulating member is defined as E, and an outerdiameter of the insulator is defined as F, then E≧F.

According to the fifth aspect of the invention, there is provided aproduction method of a gas sensor including a gas sensing element havingan gas-exposed portion, a hollow housing having a first and a second endportion, holding the gas sensing element therein so as to arrange thegas-exposed portion outside the housing for exposure to a gas to bemeasured, a protective cover installed on the first end portion of thehousing to cover the gas-exposed portion of the gas sensing element,leads connecting with the gas sensing element, extending from the secondend portion of the housing for electrical communication with an externaldevice, an insulator retaining therein the leads connecting with the gassensing element, and a metallic cover joined to the housing to hold theinsulator therein. The production method comprises the steps of: (a)preparing an assembly of the housing and the metallic cover attached toan outer wall of the second end portion of the housing; (b) rotating theassembly around a central axis thereof; (c) keeping a rotational speedof the assembly at a given constant value; and (d) emitting a laser beamto a circumference of the metallic cover of the assembly to weld themetallic cover to the housing.

In the preferred mode of the invention, the assembly is rotated with thehousing oriented upward and the metallic cover oriented downward.

A difference between an outer diameter of the outer wall of the secondend portion of the housing to be welded to the metallic cover and aninner diameter of the metallic cover, that is, the inner diameter of themetallic cover minus the outer diameter of the housing falls within arange of −0.15 mm to 0.1 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinbelow and from the accompanying drawings of thepreferred embodiments of the invention, which, however, should not betaken to limit the invention to the specific embodiments but are for thepurpose of explanation and understanding only.

In the drawings:

FIG. 1 is a longitudinal sectional view which shows a gas sensoraccording to the present invention;

FIG. 2(a) is a side view which shows an insulator;

FIG. 2(b) is a sectional view taken along the line 2 b—2 b in FIG. 2(a);

FIG. 2(c) is a sectional view taken along the line 2 c—2 c in FIG. 2(a);

FIG. 3(a) is a longitudinal sectional view which shows installation ofan insulator in a cover assembly in a prior art structure;

FIG. 3(b) is a longitudinal sectional view which shows installation ofan insulator in a cover of a gas sensor of the invention;

FIG. 4(a) is a plan view which shows an elastic member used in retainingan insulator;

FIG. 4(b) is a sectional view taken along the line 4 b—4 b in FIG. 4(a);

FIGS. 5 and 6 are sectional views which show a sequence of installationprocesses of installing an insulator in a cover;

FIG. 7 is a partially enlarged view of FIG. 6;

FIG. 8 is a sectional view which shows an insulator installed in a coverin the installation processes shown in FIGS. 5 and 6;

FIG. 9(a) is a partially sectional view which shows installation of acover on a housing in a prior art structure;

FIG. 9(b) is a partially sectional view which shows installation of acover on a housing in a prior art structure;

FIG. 10(a) is a perspective view which shows a metal ring used inholding a sensing element;

FIG. 10(b) is a cross sectional view taken along the line 10 b—10 b inFIG. 10(a);

FIG. 11(a) is a perspective view which shows a metal ring used in theprior art structure as shown in FIG. 9(a);

FIG. 11(b) is a cross sectional view taken along the line 11 b—11 b inFIG. 11(a);

FIG. 12 is a partially sectional view which shows installation of acover and a cover assembly on a housing;

FIG. 13 is a partially sectional view which shows a weld of a housingand a cover;

FIG. 14(a) is a perspective view which shows a terminal leading to anouter electrode of a sensing element;

FIG. 14(b) is a perspective view which shows a terminal leading to aninner electrode of a sensing element;

FIG. 15(a) is a partially sectional view which shows arrangement ofleads in an upper portion of a cover in the prior art structure shown inFIG. 9(a);

FIG. 15(b) is a partially sectional view which shows arrangement ofleads in an upper portion of a cover in a gas sensor of the invention;

FIG. 16 is a perspective view which shows a heater disposed in a sensingelement;

FIG. 17 is an illustration which shows a welding process of joining acover to a housing;

FIG. 18(a) is a plan view which shows the first modification of anelastic member used in retaining an insulator;

FIG. 18(b) is a sectional view taken along the line 18 b—18 b in FIG.18(a);

FIG. 19(a) is a plan view which shows the second modification of anelastic member used in retaining an insulator;

FIG. 19(b) is a sectional view taken along the line 19 b—19 b in FIG.19(a);

FIG. 20(a) is a plan view which shows the third modification of anelastic member used in retaining an insulator;

FIG. 20(b) is a sectional view taken along the line 20 b—20 b in FIG.20(a);

FIG. 21(a) is a plan view which shows the fourth modification of anelastic member used in retaining an insulator; and

FIG. 21(b) is a sectional view taken along the line 21 b—21 b in FIG.21(a).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like numbers refer to like partsin several views, particularly to FIG. 1, there is shown a gas sensor 1according to the present invention which may be employed in automotiveair-fuel ratio control systems to measure an oxygen content in exhaustgasses of an internal combustion engine.

The gas sensor 1 generally includes a sensing element 10 and a hollowcylindrical metallic housing 4 having disposed therein the sensingelement 10 hermetically. The housing 4 also serves as a sensor mount formounting the gas sensor 1, for example, in an exhaust pipe of thevehicle. The sensing element 10 has a gas-exposed portion 11 exposed toa gas to be measured in a gas chamber 13 defined by a protective coverassembly 2. The gas sensor 1 also includes leads 81 and 91, an insulator5, and a metallic cover 3. The leads 81 and 91 connect with the sensingelement 10 through connectors 75 and 76. The insulator 5 is retained inthe metallic cover 3 and holds therein ends of the leads 81 and 91.

A cover 39 is installed on an upper portion of the metallic cover 3through a cylindrical water-repellent filter 57 by crimping. The cover39 and the metallic cover 3 have formed therein first and second airvents 58 and 59 through which the air is introduced into a reference gaschamber 12 defined within the gas sensing element 10 through thewater-repellent filter 57.

The insulator 5 consists of a hollow cylindrical body 51 retaining theleads 81 and 91 in an end thereof and a flange 52 greater in diameterthan the body 51.

The metallic cover 3 consists of a small-diameter portion 31, alarge-diameter portion 32, and a shoulder portion 33 connecting thesmall-diameter portion 31 and the large-diameter portion 32. Thesmall-diameter portion 31 has the inner diameter which is greater thanthe outer diameter of the cylindrical body 51 and smaller than the outerdiameter of the flange 52. The large-diameter portion 32 has the innerdiameter which is greater than the outer diameter of the flange 52. Themetallic cover 3 is joined at an end directly to the housing 4.

The insulator 5 is installed in the metallic cover 3 by pressing thesurface 522 of the flange 52 through an elastic member 6 fitted into thelarge-diameter portion 32 to bring the surface 521 of the flange 52 intoconstant engagement with the shoulder portion 33. Specifically, theinsulator 5 is retained by the elastic member 6 and the shoulder 33 ofthe metallic cover 3.

The insulator 5, as clearly shown in FIG. 2(a), includes the cylindricalbody 51 and the flange 52 greater in diameter than the cylindrical body51. The insulator 5 also includes a lower cylindrical portion 53 whichis slightly greater in diameter than the cylindrical body 51. Thecylindrical body 51 has, as shown in FIGS. 1 and 2(b), formed thereinfour holes 511, 512, 513, and 514 through which the leads 81 and 91connecting with the sensing element 10 and a pair of leads 171connecting with a heater 15, as will be described later, pass,respectively. The flange 52 and the lower cylindrical portion 53 have,as shown in FIG. 2(c), formed therein a cylindrical bore 531communicating with the holes 511 to 514.

The metallic cover 3, as clearly shown in FIGS. 1 and 3(b), consists ofthe small-diameter portion 31, the large-diameter portion 32, and theshoulder portion 33. The small-diameter portion 31 has the sizesufficient for the cylindrical body 51 of the insulator 5 to be disposedtherein. The large-diameter portion 32 has the size sufficient for theflange 52 of the insulator 5 to be disposed therein.

The elastic member 6 is, as clearly shown in FIGS. 4(a) and 4(b), anexternally serrated lockwasher which consists of an annular plate 61,six lock tabs 62, and six guide tabs 67. The annular plate 61 is smallerthan the inner diameter of the large-diameter portion 32 of the metalliccover 3. The lock tabs 62 project from the outer periphery of theannular plate 61 so as to have the outer diameter greater than the innerdiameter of the large-diameter portion 32. Each of the guide tabs 67 isformed between adjacent two of the lock tabs 62 for facilitating theease of insertion of the elastic member 6 into the metallic cover 3 andincreasing the rigidity of the elastic member 6. It is advisable thatthe number of the lock tabs 62 be greater than or equal to three (3).When the elastic member 6 is installed in the metallic cover 3, it is,as clearly shown in FIG. 1, inserted with the lock tabs 62 all facingtoward an opening of the metallic cover 3.

The installation of the insulator 5 in the metallic cover 3 is achievedusing the elastic member 6 in the following manner.

First, the elastic member 6 and the insulator 5 are, as shown in FIGS. 5and 6, set on a jig 69. The jig 69 has a central cylinder 691 projectingupward, as viewed in the drawings. The central cylinder 691 has formedin an end thereof an annular groove 692 into which the lower cylindricalportion 53 of the insulator 5 is to be fitted. An outer end wall 693 ofthe central cylinder 691 defining the annular groove 692 is, as can beseen in FIG. 6, designed to have the width so that an end of the endwall 693 may come into contact only with the annular plate 61 of theelastic member 6 without interfering with the tabs 62 and 67.

Next, the metallic cover 3 is, as shown in FIGS. 6 and 8, put on theinsulator 5 and the elastic member 6 set on the jig 69 and forced, ascan be seen in FIG. 8, into a cylindrical chamber 695 defined by theouter wall of the central cylinder 691 within the jig 69, therebyestablishing elastic tight engagement of the lock tabs 62 of the elasticmember 6 with the inner wall of the metallic cover 3.

The lock tabs 62 of the elastic member 6 are bent at a given obtuseangle away from the surface 521 of the insulator 5. This structurecauses the elastic member 6 to be pressed inward when forced into thelarge-diameter portion 32 of the metallic cover 3, thereby producing thepressure which urges the flange 52 of the insulator 5 into constantengagement with the shoulder 33 of the metallic cover 3.

FIG. 3(a) shows, as one example, installation of the insulator 95 in theconventional gas sensor 9. The insulator 95 is substantially identicalin structure with the insulator 5 in the embodiment, as described above,and has the flange 952. The insulator 95 is disposed within the outercover 932. The inner cover 931 is fitted partly within the outer cover932 with an end 934 urging the flange 952 of the insulator 95 intoconstant engagement with the shoulder 935 of the outer cover 932 throughthe spring 956.

Comparison between FIGS. 3(a) and 3(b) shows that the use of the elasticmember 6 in the gas sensor 1 of the invention eliminates the need forthe inner cover 931 employed in the conventional gas sensor 9 and thatthe length L1 between the shoulder portion 33 and the lower end of themetallic cover 3 may be determined more accurately than that between theshoulder 935 and the lower end of the inner cover 931.

The elimination of the need for the inner cover 931 allows the innerdiameter of the metallic cover 3 to be increased so that the distanceL2, as shown in FIG. 9(b), between the metallic cover 3 and the lead 711of the signal pickup terminal 71 communicating electrically with ameasuring electrode, as will be described later in detail, of thesensing element 10 may be increased as compared with that in theconventional gas sensor 9 shown in FIG. 9(a).

The housing 4, as shown in FIGS. 1 and 9(b), has a chamber 40, a crimpedend 41, and an annular seat 49. The annular seat 49 is formed on aninner wall of the housing 4. The sensing element 10 is retained withinthe chamber 40 tightly by crimping the end 41 of the housing 4 to pressa metallic ring 412, a cylindrical insulator 411, and a seal member 413downward, as viewed in the drawings, to urge a central flange 109 of thesensing element 10 into constant engagement with the seat 49 through apacking 18.

The conventional gas sensor 9 has, as shown in FIG. 9(a), a housing 94.The housing 94 has an end 940 crimped to press a metallic ring 946 tohold a sensing element 910 within the housing 94 through an insulator943, a pad 942, and a seal member 941. Between the metallic ring 946 andthe insulator 943, a flange 939 of the inner cover 931 is interposed tosecure the inner cover 931 on the housing 94. A packing 918 is disposedbetween the sensing element 910 and an inner wall of the housing 94.

As will be apparent from comparison of the structure of the gas sensor 1in FIG. 9(b) with that of the conventional gas sensor 9 in FIG. 9(a),the crimped end 41 of the housing 4 is sheathed with the metallic cover3, thereby resulting in greatly improved corrosion resistance of the end41.

The conventional gas sensor 9, as discussed above, has the flange 939 ofthe inner cover 931 retained on the seal member 941, the pad 942, andthe insulator 943 within the housing 94. The seal member 941 is made oftalc compressed in a production process of the gas sensor 1. Thus, ifthere is a variation in volume of the seal member 941, it will cause thelocation of the flange 939 to be changed, which results in a change involume of a space within which the metallic ring 946 is to be disposed,thus requiring adjustment of the thickness of the metallic ring 946 forcrimping the end 940 of the housing 94 completely. The change inlocation of the flange 939 will also result in a variation in overalllength of the gas sensor 9. Compensating for this variation requiresadjustment of the spring 956 disposed between the insulator 95 and theouter cover 932. In contrast, the gas sensor 1 of this embodiment hasthe metallic cover 3 bonded directly to the housing 4. The overalllength of the gas sensor 1 is, thus, insensitive to a variation involume of the seal member 413 made of compressed talc, therebyeliminating the need for adjustment of the thickness or size of anyparts.

The metallic ring 412 of the gas sensor 1 is, as shown in FIGS. 10(a)and 10(b), made by looping a given length of a round bar and has a gap418. The metallic ring 941 of the gas sensor 9 is, as shown in FIGS.11(a) and 11(b), made by machining a jointless round strip member havingan oval cross section. This difference in structure allows productioncosts of the metallic ring 412 to be reduced, resulting in a decreasedtotal costs of the gas sensor 1. The use of the metallic ring 412 isrealized with the improvement of airtight sealing established by weldingthe metallic cover 3 to the whole of a circumference of the housing 4.

The protective cover assembly 2 is, as clearly shown in FIG. 12,retained tightly in an annular groove 420 formed in the bottom of thehousing 4 by crimping an annular extension or skirt 421 inward. Theprotective cover assembly 2 consists of outer and inner cylindricalcovers 21 and 22 which have ends bent outward to form flanges 219 and229, respectively. The inner cover 22 has an open end bulged so as to beinstalled in the annular groove 420 in a suitable fit with a guide wall422. The installation of the protective cover assembly 2 on the housing4 is achieved by putting the flanges 219 and 229 in the grooves 420together and bending the skirt 421 inward tightly.

The metallic cover 3 is, as clearly shown in FIGS. 1 and 12, mounted atan end on the housing 4 in constant contact with an annular step 48formed on a flange 64 and joined at a circumferential portion 300 to thewhole of a periphery of an upper portion of the housing 4 by, forexample, laser welding. This structure improves the airtight sealingbetween the housing 4 and the metallic cover 3 as compared with aconventional structure such as the one shown in FIG. 9(a) in which theinner cover 931 is installed at an end thereof in the housing 94.

The step 48 is machined on an upper surface of the flange 64 to have aflat surface for facilitating establishment of concentricity of themetallic cover 3 and the housing 4 and locating portions of the metalliccover 3 and the housing 4 to be welded to each other accurately.

FIG. 13 shows a cross section of a weld 34 of circumferential portionsof the metallic cover 3 and the housing 4 which were fused by the laserwelding and then solidified. The weld 34 is, as can be seen from thedrawing, made up of a wider half-moon portion 341 formed in the metalliccover 3 and a narrower semi-oval portion 342 formed in the outer wall ofthe housing 4. If maximum widths of the half-moon portion 341 and thesemi-oval portion 342 are defined as A and B, respectively, the depth ofthe semi-oval portion 342 is defined as D, and the thickness of themetallic cover 3 is defined as T, conditions of B≧0.6 A and D≧T aresatisfied. In this embodiment,A is 1.0 mm. B is 0.6 mm. D is 0.6 mm. Tis 0.6 mm. This results in an improved strength of the joint of themetallic cover 3 and the housing 4.

The sensing element 10 consists of a cup-shaped solid electrolyte bodywhich has formed therein a chamber 100. Within the chamber 100, theheater 15 is disposed for heating the sensing element 10 up to a givenoperating temperature. The chamber 100 defines a reference gas chamber12 communicating with the air vents 58 and 59. The sensing element 10has outer and inner electrodes 130 and 120 both made of Pt. The outerelectrode 130 is attached to the gas-exposed portion 11 and functions asa measuring electrode, while the inner electrode 120 is attached to aninner wall of the sensing element 10 and functions as a referenceelectrode. In operation, the electromotive force is produced between theouter and inner electrodes 130 and 120 as a function of theconcentration of a gas within the gas chamber 13 and outputted throughthe leads 81 and 91.

For the operation of the gas sensor 1 in more detail, reference is madeto U.S. application Ser. No. 09/196,693, filed on Nov. 20, 1998,assigned to the same assignee as that of this application, disclosure ofwhich is incorporated herein by reference.

The signal pickup terminal 71, as shown in FIGS. 1 and 14(a), is mountedon the outer wall of the sensing element 10 in electrical connectionwith the outer electrode 130. The signal pickup terminal 72, as shown inFIGS. 1 and 14(b), is fitted in the chamber 100 of the sensing element10 in electrical connection with the inner electrode 120 and has acylindrical holder 725 which holds the heater 15 therein. The signalpickup terminals 71 and 72 have, as already described, the leads 711 and721 electrically connecting with the leads 81 and 91 through theconnectors 75 and 76. The signal pickup terminals 71 and 72 are eachmade of a heat-resisting spring steel such as INCONEL (trade mark) whosemain component is Ni for improving the durability.

The heater 15, as shown in FIG. 16, includes a ceramic square rod 150having a rectangular cross section. The ceramic square rod 150 is madeof a laminate of substrates each formed with a ceramic sheet and heatgenerating members. Metallic terminal plates 153 are bonded to opposedsurfaces of the ceramic square rod 150 in electrical connection with theheat generating members through the ceramic substrates, respectively.The metallic terminal plates 153 each have formed thereon conductivepins 154 connecting with the leads 171.

A rubber bush 45, as clearly shown in FIG. 15(b), is installed in an endof the small-diameter portion 31 of the metallic cover 3. The rubberbush 45 retains therein the leads 81, 91, and 171. The cover 39 is, asdescribed above, installed on the small-diameter portion 31 of themetallic cover 3 by crimping. If the outer diameter of the rubber bush45 is defined as E, and the outer diameter of the cylindrical body 51 ofthe insulator 5 is defined as F, then E≧F. In this embodiment, E=10.1mm. F=9.8 mm. This structure results substantially in agreement of theinterval d between opposed two of four holes in the rubber bush 45through which the leads 81, 91, and 171 pass with the interval D betweenopposed two of the holes 511 to 514 in the insulator 5.

In the conventional gas sensor 9 as shown in FIG. 15(a), the outerdiameter E of a rubber bush 945 is smaller than the outer diameter F ofthe insulator 95. Specifically, E is 6.5 mm, and F is 9.8 mm. Thisstructure results in a difference between the intervals d and D whichwill require bending the leads 981 and 991 extending from the insulator95 inward when they are inserted into the rubber bush 945 in anassembling process. It is not advisable that the leads 981 and 991 bebent at sharp angles, thus requiring increase in interval L3 between theinsulator 95 and the rubber bush 945 which will result in increase inoverall size of the gas sensor 9. In contrast, the structure of the gassensor 1 shown in FIG. 15(b) allows the interval L3 between theinsulator 5 and the rubber bush 45 to be decreased, so that the overallsize of the gas sensor 1 can be reduced greatly as compared with the gassensor 9.

The heater 15 may alternatively be made of a ceramic round bar. Thesensing element 10 may alternatively be made of a laminated plateelement. For example, U.S. Pat. No. 5,573,650, issued Nov. 12, 1996 toFukaya et al., teaches such a structure of the sensing element 10,disclosure of which is incorporated herein by reference.

FIG. 17 shows a process of welding the metallic cover 3 and the housing4 together according to the second embodiment of the invention.

First, the metallic cover 3 is fitted on the housing 4 to form anassembly 100. Next, the assembly 100 is turned about a longitudinalcenter line 101 thereof. After a given rotational speed is reached andkept constant, a welding gun 80 is activated to emit a laser beam 88 tothe circumferential portion 300 of the metallic cover 3.

Specifically, the outer wall of the housing 4 to be welded to themetallic cover 3 is so machined as to have the diameter greater than theinner diameter of the metallic cover 3 by 0.1 mm. The assembly 100 isformed by pressing the housing 4 into the metallic cover 3.

The difference between the outer diameter of the outer wall of thehousing 4 to be welded to the metallic cover 3 and the inner diameter ofthe metallic cover 3 (i.e., the inner diameter of the metallic cover 3minus the outer diameter of the housing 4) may be within a range of−0.15 mm to 0.1 mm, preferably within a range of −0.10 mm to 0.05 mm forproviding for ease of assembly and hermetic sealing between the metalliccover 3 and the housing 4.

Next, a welding jig 85 is provided which consists of a supporting plate851 and a rotary plate 852. The metallic cover 3 of the assembly 100 isinstalled rotatably in the supporting plate 851, while the housing 4 isfixed in the rotary plate 852 so that the metallic cover 3 may beoriented downward, while the housing 4 may be oriented upward.

The rotary plate 852 is rotated using, for example, an electric motor(not shown) to turn the assembly 100. When the circumferential speed ofthe assembly 100 reaches 1500 mm/minute, it is kept constant. Thewelding gun 80 is turned on to emit the laser beam 88 to join themetallic cover 3 and the housing 4 together.

The laser beam 88 may be emitted either continuously or intermittently.The circumferential speed of the assembly 100 may be set to anothervalue within a range not sacrificing the welding strength and weldedconditions of the metallic cover 3 and the housing 4.

FIGS. 18(a) and 18(b) show a modification of the elastic member 6

The elastic member 6 is different from the one shown in FIGS. 4(a) and4(b) only in that the lock tabs 62 and the guide tabs 67 all lie flushwith the annular plate 61. In other words, the elastic member 6 hasopposed surfaces symmetrical with each other.

FIGS. 19(a) and 19(b) show the second modification of the elastic member6.

The elastic member 6 has the lock tabs 62 bent alternately in oppositedirections. In this modification, opposed surfaces of the elastic member6 are, like the above first modification, symmetrical with each other,which will allow a workman to place the elastic member 6 on the jig 69,as shown in FIG. 6, in installation of the insulator 5 in the metalliccover 3 without having to pay attention to orientation of the elasticmember 6.

FIGS. 20(a) and 20(b) show the third modification of the elastic member6 which is different from the one shown in FIGS. 4(a) and 4(b) only inthat the lock tabs 62 are all bent in the same direction at an angle of45° or more to the surface of the annular plate 61.

FIGS. 21(a) and 21(b) show the fourth modification of the elastic member6 which is different from the one shown in FIGS. 4(a) and 4(b) only inthat two protrusion 618 are formed on the diametrically opposed guidetabs 67 as a mark which a workman uses in determining which surface ofthe elastic member 6 is to be oriented to the inside of the metalliccover 3 when installing the insulator 5 in the metallic cover 3.

Specifically, the protrusions 618 extend in the same direction as thatin which the lock tabs 62 are bent so as not to interfere with theinsulator 5 when installed in the metallic cover 3. The number of theprotrusions 618 is not limited to two (2), and at least one protrusion618 may be provided on the elastic member 6. In either of the third andfourth modifications, it becomes easy for a workman to visually perceiveone of opposed surfaces of the elastic member 6 to be oriented towardthe inside of the metallic cover 3 in installation of the insulator 5.To this end, it is also advisable that either of the opposed surfaces ofthe elastic member 6 be colored or that the opposed surfaces be paintedin different colors.

While the present invention has been disclosed in terms of the preferredembodiments in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. A gas sensor comprising: a gas sensing elementhaving an gas-exposed portion; a hollow housing having a first and asecond end, said housing holding said gas sensing element therein so asto arrange the gas-exposed portion outside said housing for exposure toa gas to be measured; a protective cover installed on the first end ofsaid housing to cover the gas-exposed portion of said gas sensingelement; leads connecting with said gas sensing element, extending fromthe second end of said housing for electrical communication with anexternal device; a metallic cover installed on the second end of saidhousing, said metallic cover including a small-diameter portion, alarge-diameter portion greater in diameter than the small-diameterportion, and a shoulder portion connecting the smaller-diameter portionand the large-diameter portion; and an insulator including a body and aflange projecting from the body, having disposed therein said leadsconnecting with said gas sensing element, the body having an outerdiameter smaller than an inner diameter of the small-diameter portion ofsaid metallic cover and being disposed within the small-diameterportion, the flange having a first and a second end surface opposed toeach other and an outer diameter which is smaller than an inner diameterof the large-diameter portion of said metallic cover and which isgreater than the inner diameter of the small-diameter portion, saidinsulator being disposed in said metallic cover with the first endsurface of the flange urged by an elastic member to bring the second endsurface into constant engagement with an inner wall of the shoulderportion of said metallic cover.
 2. A gas sensor as set forth in claim 1,wherein said metallic cover has a given length, and wherein the elasticmember is so designed as to produce a first pressure acting on an innerwall of the large-diameter portion of said metallic cover in a radialdirection of the large-diameter portion and a second pressure acting onthe second end surface of the flange of said insulator in a lengthwisedirection of said metallic cover perpendicular to the radial directionof said metallic cover.
 3. A gas sensor as set forth in claim 2, whereinthe elastic member includes an annular plate and tabs, the annular platehaving a diameter smaller than the inner diameter of the large-diameterportion of said metallic cover, the tabs projecting from the annularplate so as to establish elastic engagement with the inner wall of thelarge-diameter portion of said metallic cover.
 4. A gas sensor as setforth in claim 3, wherein the elastic member also include guideprotrusions each of which is disposed between adjacent two of the tabsand which projects from the annular plate to a circular line smallerthan the inner diameter of the large-diameter portion of said metalliccover.
 5. A gas sensor as set forth in claim 3, wherein the elasticmember is made of a plate member having opposed surfaces which aresymmetrical with each other.
 6. A gas sensor as set forth in claim 3,wherein the tabs of the elastic member are so designed that when saidinsulator is inserted into said metallic cover, some of the tabs arebent elastically in a first direction away from one of opposed surfacesof the annular plate by elastic pressure produced by the insertion ofsaid insulator, while the other tabs are bent elastically in a seconddirection opposite the first direction.
 7. A gas sensor as set forth inclaim 3, wherein the tabs extend from the annular plate at an angle ofapproximately 45° or more to one of opposed surfaces of the annularplate.
 8. A gas sensor as set forth in claim 3, wherein the elasticmember also includes a protrusion formed on one of opposed surfacesthereof.
 9. A gas sensor as set forth in claim 3, wherein the elasticmember has opposed surfaces at least one of which is painted so that theopposed surfaces have different colors.
 10. A gas sensor as set forth inclaim 3, wherein the tabs of the elastic member are so designed thatwhen said insulator is inserted into said metallic cover, the tabs arebent elastically in the same direction away from one of opposed surfacesof the annular plate by elastic pressure produced by the insertion ofsaid insulator.
 11. A gas sensor as set forth in claim 1, furthercomprising an elastic insulating member disposed on an end of saidmetallic cover remote from said housing to retain said leads therein,and wherein if an outer diameter of the elastic insulating member isdefined as E, and an outer diameter of said insulator is defined as F,then E≧F.
 12. A gas sensor comprising: a gas sensing element having agas-exposed portion; a hollow housing having a first end and a secondend, said housing holding said gas sensing element therein so as toarrange the gas-exposed portion outside the housing for exposure to agas to be measured; a protective cover installed on the first end ofsaid housing to cover the gas-exposed portion of said gas sensingelement; leads connecting with said gas sensing element, extending fromthe second end of said housing for electrical communication with anexternal device; an insulator retaining therein said leads connectingwith said gas sensing element; a metallic cover having a given length,said metallic cover being joined directly to said housing to hold saidinsulator therein in engagement of an end of said metallic cover to saidhousing; wherein said housing has an outer wall extending between thefirst and second ends, and wherein said metallic cover is welded to thewhole of the circumferences of the outer wall of said housing; andwherein a welded portion is formed with welding of said metallic overand said housing which includes a wider portion formed in said metalliccover and a narrower portion formed in the outer wall of said housing,and wherein if maximum widths of the wider and narrower portions aredefined as A and B, respectively, the depth of the narrower portion isdefined as D, and the thickness of said metallic over 3 defined as T,conditions of B≧0.6 A and D≧T are satisfied.
 13. A gas sensor as setforth in claim 12, further comprising an elastic insulating memberdisposed on an end of said metallic cover remote from said housing toretain said leads therein, and wherein if an outer diameter of theelastic insulating member is defined as E, and an outer diameter of saidinsulator is defined as F, then E≧F.